1. The field of art to which the invention pertains includes the art of acoustics and mufflers for jet velocity silencing.
2. Mufflers for the purpose of silencing or suppressing equipment noise represent a well developed art and are available in an endless variety of forms to meet the abundance of applications for which they are needed. Basically, mufflers are of two basic types that include the reactive type in which geometric shapaes are utilized to reflect noise energy back to the source, and the dissipative type in which acoustical performance is obtained through the use of sound absorbing materials.
One form of equipment for which the use of mufflers has long been regarded as impractical and unnecessary is with accessory type safety equipment available to accommodate extenuating operating circumstances and which as a usual matter are infrequently operative. With such equipment being used so relatively seldom and its operation when used being of such relatively short duration, the cost for suppressing associated noises has long been regarded as economically unjustified. Exemplifying equipment of that type is a safety valve utilized, for example, on high pressure-high temperature steam service for relieving overpressure from steam generating equipment. Safety valves useful for those purposes are commercially available under the trademark CONSOLIDATED. It is not uncommon for the discharge from such high capacity safety valves to achieve sonic or near sonic velocity in the discharge duct leading to atmosphere such that not only is the duct flow transmitting noise created at the valve but at the same time is per se generating noise on discharging its flow to atmosphere.
With the advent of federal regulations enacted by the Occupational Safety and Health Administration (OSHA), a legal need has been created to significantly reduce the noise level generated by such discharging safety valves as follows:
______________________________________ Duration per Day Permissible Noise dB (A) ______________________________________ 8 hrs. 90 4 hrs. 95 2 hrs. 100 1 hr. 105 1/2 hr. 110 1/4 hr. 115 ______________________________________
While implementing requirements for the above may appear seemingly simple from the vast availability of muffler technology, it has been found that the use of conventional muffler approaches for those purposes has produced muffler equipment which is practically prohibitive in both size and weight without which an intolerable degree of back pressure is imposed on the valve. The former, of course, imposes a difficultly of accommodation, while the latter adversely affects performance of the valve and thereby at least partially defeats the safety purposes for which the valve was installed. Despite recognition of the foregoing problems, a ready solution has not heretofore been known.